I just installed the latest Ubuntu (Karmic Koala) and thought that I could try out using KDE as my desktop for a change. I got it mostly set up to my liking, but there are certain applications which are from the gnome-side (gtk-based) which look ugly (e.g. synaptic). They use some default look and feel for their buttons and other widgets. How can I change their themes under KDE?

hint: in system settings >> appearance >> style there is a configure option.
it allows you to configure lots of things. you can also use predefined styles, just check the 'options' in the down-left corner of qtcurve configuration.
it wil apply to both qt and gtk :)